The invention relates generally to monitoring performance of a printing system and more particularly to systems and methods for optically monitoring print performance.
lnkjet printers provide an inexpensive means for depositing print material (e.g., ink) on paper or another medium. A conventional inkjet printer includes an inkjet printhead mounted on a carriage, which moves the printhead over the paper. The printhead has an ink supply and an array of nozzles which project ink droplets in a particular pattern onto the paper. Each nozzle is formed by a nozzle chamber, a firing mechanism, and an orifice, with the firing mechanism being located within the nozzle chamber. During operation, the nozzle chamber receives a volume of ink from the ink supply, so that when the firing mechanism is activated, an ink droplet is fired from the chamber through the orifice onto the paper. In most inkjet devices, the printhead is moved from side-to-side, while a paper-advancement mechanism is used to change the position of the paper relative to the carriage.
In inkjet printers or in any other printer that deposits print material in the form of dots, the size and the placement of the dots are critical to print quality. If dots are missing or are incorrectly sized or placed, visual defects that are detrimental to the print quality may result. Such errors are noticeable as losses in resolution of the features being printed, imperfections in colors of features or areas being printed, and unintended spatial patterns that appear as Nyquist noise, mosaicing, banding, missing content, or just poor print quality in general.
Systems for detecting some of these losses are known. For example, with regard to the imperfections in colors of features or areas being printed, U.S. Pat. No. 6,036,298 to Walker, which is assigned to the assignee of the present invention, describes the use of a sensing system that includes a single monochromatic photodetecting element, such as a photodiode. The photodetecting element is mounted for travel with an inkjet printhead. A blue light emitting diode (LED) illuminates a region that is imaged by the photodetecting element. In one step, a portion of the paper having no ink is sampled by the photodetecting element to generate a bare media signal, while in a second step, the photodetecting element samples a portion of the media having ink to generate an ink signal. A controller compares the difference between the amplitudes of the bare media signal and the ink signal to a set of reference values. The comparisons determine the color of the ink at the second portion of the media. As another feature of the Walker system, test marks may be formed at specific locations on the paper. After the test marks have been formed, the sensing system may be used to verify the presence of the test marks at the desired locations. When a test mark is not found at a desired location, a xe2x80x9cNoxe2x80x9d signal may be generated to adjust the firing parameters of the print operation. As yet another aspect, color balance may be adjusted as a response to determining that a particular color has not been formed.
A second system that utilizes optical sensing within an inkjet printer is described in U.S. Pat. No. 4,328,504 to Weber et al. As in Walker, the Weber et al. sensing system utilizes a single photodetecting element and uses signal comparisons to determine whether print parameters should be adjusted. The photodetecting element is mounted to an inkjet printhead or other printing device. The photodetecting element outputs a continuous signal as it is moved adjacent to the surface of a paper having ink droplets. This output signal is compared to a signal that represents the desired signal. When a difference is detected between the output signal and the desired signal, a correction is initiated. For example, if a pulse that is generated as a result of detecting an ink droplet along the paper has a duration that is different than the duration of the corresponding pulse along the desired signal, it is presumed that the size of the droplet is incorrect and a correction is triggered.
While the prior art systems and methods operate well for their intended purposes, there are limitations regarding the ability to monitor print quality control. The limitations are tolerable for most low demand print operations, such as printing text documents from a word processing program. However, as the complexity of a document to be printed increases, so do the demands that are placed on the print parameters that relate to print quality. For example, user expectations during the printing of a digital photograph or an image from the World Wide Web have reached a level that requires a printer to be operating at or near peak performance. The known approaches to monitoring print operations may not be sufficient in some applications.
What is needed is a print monitoring approach that enables the acquisition of a richness of print-quality related information, so that quality control can be maintained at a high level.
The invention utilizes a print monitoring approach in which high resolution two-dimensional frames of image information are captured to resolve physical characteristics of individual droplets that have been deposited on a medium, such as a sheet of paper. An optical detector having a two-dimensional array of closely spaced sensor elements is mounted for movement with a print assembly, such as an inkjet printhead, that deposits the droplets on the medium. A processor is responsive to the optical detector to adjust print quality parameters when the physical characteristics or features of the imaged droplets are detected as being outside of a range of acceptability. The physical features that are imaged and used to generate physical characteristic feedback information may include information regarding the gyrational pattern that is formed as a consequence of the droplet striking and settling on the medium. Alternative feedback information includes data related to the centroid of a droplet, data related to the position of peak light absorption by the droplet, and data related to the intersection of two principal diameters.
In the embodiment in which the print assembly is an inkjet printhead, the dots that are formed by droplets on the medium typically have a diameter in the range of 20 xcexcm to 60 xcexcm. Imaging optics are selected to provide a high resolution, but there is concern that, given available illumination devices and required response time, the optics will have a diffractive limit (e.g., 35 xcexcm) that does not quite reach a preferred level. To provide compensation for resolution in a first direction, the adjacent columns of sensor elements within the optical detector may be offset in an orthogonal direction to the movement of the print assembly and of the optical detector relative to the medium. The measure of the column-to-column offset should be less than the measure of the pitch of sensor elements within a column. For example, if there are six columns of sensor elements within the optical detector, the offset may be one-sixth of the pitch of sensor elements within the columns. Any measure of offset that is less than the pitch will aid in providing sufficient resolution in the first direction to detect useful physical characteristics of the individual droplets on the medium. The diffractive limit of resolution of imaging optics may be countered in the second (orthogonal) direction parallel to the relative movement by acquiring image frames at a rate sufficiently high to limit droplet displacement in successive images to a distance similarly less than the pitch of the columns.
The optical detector arrangement may also include at least one source of illumination. The source may be a light emitting diode (LED) that provides illumination at an angle in the approximate range of 20 degrees to 65 degrees relative to the normal of the surface of the medium on which the droplets are deposited. For embodiments in which the xe2x80x9cdot gainxe2x80x9d is to be measured, a number of LEDs may be mounted and sequentially activated to illuminate the field of view of the optical detector at different angles of incidence, thereby allowing the shift in the position of the droplet centroid to be sensed.
A combination of a cylindrical lens and a prism may be used to direct light from the illumination source to the medium at the desired angle of incidence. The optical detector arrangement also includes optical lens elements. A droplet-imaging optical device is positioned between the medium and the array of sensor elements. The optical device may provide demagnification, but this is not critical. Any of the optics may be diffractive limited.
In operation of the invention, at least one dot is formed on a medium, such as a sheet of paper, by projecting a droplet of print material onto the medium. The two-dimensional array of sensor elements and the high frame rate achieve a sequence of frame image information to provide (with image processing) a sufficient resolution to enable the processor to resolve detailed information regarding the physical characteristics of the dot. For example, gyrational information or different types of droplet position information (e.g., the position of the dot-centroid) may be identified. The information can then be used to determine whether adjustments should be made to the print operation parameters.
The invention may be used to inspect xe2x80x9cstealthyxe2x80x9d dots. The term xe2x80x9cstealthy dotxe2x80x9d is defined herein as a dot that is purposely printed using a single nozzle, where the dot is printed in a region of a medium in which no final content is intended or in a region that is intended to be covered over after the specific dot has been inspected. The concept of xe2x80x9cstealthyxe2x80x9d derives from the fact that the conventional inkjet printers print dots that are so small individually that the isolated dots are virtually undetectable by the unaided human eye. Stealthy dots or other stealthy marks may be used to accurately determine the position of the print assembly (e.g., inkjet printhead) on the medium. For example, stealthy dots may be used to align the print assembly to add print content to designated areas of a medium having previously printed material. This application is particularly suitable for adding content to pre-printed forms. Stealthy dots may also be used to enable detection of the top, bottom or edges of the medium. Thus, printing onto a page can occur with accurate full-bleeds or with accurate margining. If desirable, the correlation of successive images acquired by the optical detector may be used to track and/or verify relative speeds and positions between the optical detector and the medium of interest. Speed and position may be determined in either or both of the directions from top-to-bottom or from side-to-side across a page. By precisely pre-positioning stealthy dots or other stealthy marks on the medium, the navigation of the optical detector (and therefore the print assembly) may be determined without using conventional encoders for carriage and paper-feed position tracking.